Numerous telecommunication system operators provide broadcast television and other types of programming to subscribers over a coaxial cable or other medium terminating at the subscriber location. Typically, the system operator will transmit different information streams to subscribers in each of multiple frequencies. For example, one frequency may carry a stream of digital data packets for one group of programming services, another frequency may carry a different information stream having digital data packets for a different group of services, etc. A service may be a cable television service (e.g., HBO), a broadcast network (e.g., NBC), a local television station, an audio-only music service, or any of various other service types. Each service is usually assigned a channel number. Because a channel number can be arbitrarily assigned to a service independently of the frequency carrying the data packets of that service, such a channel number is often referred to as a “virtual” channel.
When a user selects a virtual channel corresponding to a desired service, a Set Top Terminal (STT) or other receiving device must know the frequency carrying the information stream that includes data packets for that service. Accordingly, the system head end transmits a Virtual Channel Table (VCT) that maps each virtual channel to the frequency on which the service data corresponding to that channel is carried. The VCT is periodically transmitted in-band among program data packets on multiple frequencies. System operators routinely move data for virtual channels to different frequencies because of equipment issues, for network management, and for various other reasons. An STT or other receiving device thus monitors an information stream on a tuned frequency for a new VCT. If a new VCT is received, it is stored and used for subsequent virtual channel selections.
Because a revised VCT mapping channels to different frequencies is not transmitted until after channels have actually been moved, problems can occur. As one example, a user may currently be watching service X corresponding to channel 10 carried on frequency Y. The user's receiving device is thus tuned to frequency Y and extracting data packets associated with channel 10 from the information stream carried on frequency Y. If the system operator moves data for service X/channel 10 to frequency Z, the user's receiving device will still be tuned to frequency Y but will be unable to find data corresponding to channel 10. Until a new VCT is received, the receiving device will not know where to find data for channel 10. Moreover, the head-end may stop transmitting VCTs in frequency Y after channel 10 is moved to frequency Z. For example, frequency Y may be reassigned for a different use (e.g., as a Data over Cable System Interface Specification (DOCSIS) carrier) or may be taken out of use (e.g., because of an equipment failure). The receiving device may then be forced to enter a search mode and try to find a VCT on another frequency. In particular, the device will search for a new VCT by sequentially tuning to each of multiple frequencies for a predetermined amount of time until a new VCT is found. In some cases, this process could take 10 minutes or more.
One existing solution is to also transmit the VCT on a separate out-of-band (OOB) frequency that does not change. A STT or other receiving device can then include a second tuner that stays tuned to that OOB frequency. This solution requires additional hardware components, however, and increases costs.